Antigen specific T-cell activation and the initiation of an immune response depend initially on the interaction of the T-cell receptor (TCR) complex with the peptide/major histocompatibility complex (MHC) present on antigen presenting cells (APC). B7 molecules, B7-1 and B7-2, are molecules which are present on APCs. A second “costimulatory” signal, provided by the interaction of B7-1 and B7-2 on the APC with their ligands CD28 and CTLA4 on T-cells, is required to complete T-cell activation and the subsequent regulation of an immune response. A need exists to regulate the B7-1 and B7-2 pathway, referred to as the B7:cD28/CTLA4 pathway. A further need exists to develop treatments for diseases that are affected by this pathway.